In recent years, a substantial amount of effort has been devoted to improving short circuiting arc welding by controlling portions of a welding cycle constituting a short circuit condition followed by an arcing condition. During the short circuit condition, a molten metal ball formed on the end of the advancing welding wire engages the molten metal pool on the workpiece causing a high current flow through the consumable welding wire and molten metal ball. This short circuit condition is terminated by an electrical pinch action causing the metal forming the molten ball on the wire to electrically constrict and then break away from the welding wire in an explosion type action often referred to as a "fuse" or "the fuse". Controlling current flow during the short circuit portion of the welding cycle is accomplished by the power supply control circuit. In addition, a premonition circuit is usually provided so that a given increase in dv/dt signals the imminent formation of the fuse. Consequently, the welding current can be dropped to a background level I.sub.B or lower immediately before the fuse occurs. In this fashion, the energy of the fuse during each welding cycle is drastically reduced. This reduces spatter at the termination of the short circuit condition. Various circuits for controlling the current flow during the short circuit portion or condition of the welding cycle are known in the art as spatter control circuits since the fuse is considered to be the primary source of spatter during short circuiting arc welding. In applicant's patents and application, incorporated by reference herein, other spatter producing dynamics of the welding process were recognized and prevented or modified by novel control concepts. One concept was to provide a high energy pulse following a slight time delay after the fuse so that the arcing condition subsequent to the fuse could be initiated by a high energy current pulse sometimes referred to as a "plasma boost" pulse. By using a high energy plasma boost current pulse immediately upon initiation of an arcing condition in the welding cycle, melting by anode heating at the tip of the welding wire being fed toward the molten metal pool on the workpiece occurred rapidly. This rapid melting allowed formation of a molten metal ball on the end of the wire of uniform size which was then moved toward the pool of molten metal as the wire was fed toward the workpiece. After the plasma boost pulse of current, a background current I.sub.B was passed through the arc to maintain the molten condition of the molten ball. By controlling the current and using a fixed time for the plasma boost pulse, the energy in the plasma boost pulse was regulated. The end of the wire was melted to form a molten metal ball having a somewhat uniform size based upon an amount of energy applied during the plasma boost current pulse. Thereafter, the arc was operated at a background current level maintaining a molten condition until the short circuit occurred.
By using a plasma boost pulse having a fixed time, a different amount of energy was introduced into the molten metal ball as the stick-out of the consumable electrode or welding wire varied. Thus, prior systems employing fixed time in the plasma boost current pulse could be used for automatic welding; however, semi-automatic welding wherein manual manipulation changed the extension presented difficulty. The plasma boost current pulse sometimes did not create enough heating on the end of the wire for melting. This caused stubbing. In addition, the duration of the welding cycle was not constant over long periods of time since there was substantial variations in the initiation of the short circuit condition of the individual cycles.
A unique driving system for the spatter control circuit has been developed by applicant wherein the individual welding cycles have a generally fixed frequency of repetition, such as 30-100 welding cycles per second. The power supply for the spatter control includes means for applying a succession of input current pulses across the wire and workpiece at a pulse frequency substantially greater than the generally fixed frequency of repetition of the welding cycles and pulse width changing means are provided for adjusting the current flow between the wire and workpiece many times during each of the welding cycles. In practice, the repetition is approximately 20 KHz so that the actual current flow during the welding cycle is adjusted at the rate determined by the period of a 20 KHz control signal. In this manner, accurate control is maintained without substantial interference with circuit parameters. When using this concept, a relatively low inductance is employed across the output leads of the power supply system. The low inductance allows the welding current to track the desired current profile during both the short circuit and the plasma stages. However, the low inductance does not produce sufficient current to consistently maintain the arc. The pulse width change means comprises a feedback control means for changing the pulse width of the input current pulses in the direction to maintain a preselected electrical characteristic. Selectively actuated circuit means are used to change the electrical condition during various portions of each of the welding cycles. In this fashion, the feedback control can be shifted during various portions of the welding cycle to cause the current control to follow a preselected pattern or profile to accomplish the electrical parameters of the spatter control circuit as previously described.
The driving circuit for the spatter control system or spatter control circuit includes a pulse width modulator that is adjusted rapidly, such as at a frequency of 20 KHz. The width of the individual pulses during the various portions of the welding cycle are controlled by the feedback control circuit which tends to maintain a preselected condition at a control point. In this manner, this control point can be biased and subject to various electrical parameters during each welding cycle to adjust the profile of the welding cycle in accordance with any plan. Thus, the PINCH cycle can be controlled by current and have a different current during different portions. The plasma boost can be controlled by a constant wattage, constant current or constant voltage feedback arrangement with a preselected profile for the selected, controlled electrical characteristic. Thus, by using the high frequency and the selectively adjustable control concept a variety of individual welding cycles can be preprogrammed and controlled.
In normal short circuiting welding operations, a relatively large choke is provided in the welding circuit to control the welding current; however, such chokes are relatively expensive, heavy and provide substantial inductive reactance in the welding circuit. This inductive reactance, when employing a high frequency driver, reduces the capability of the welding circuit to follow the preselected profile determined by the command control circuit. In addition, a relatively large choke reduces the time required for a welding current to be reduced to an acceptable level preparatory to the fuse explosion at the end of the PINCH portion of the welding cycle. In prior applications, it has been suggested to place a resistor across the switch operated by the premonition circuit so that as the dv/dt or di/dt indicates an upcoming fuse explosion in the PINCH cycle, the switch is opened and the resistor is placed in series with the choke. This drastically reduces the welding current to decrease the amount of energy at the fuse explosion. However, this arrangement employs relatively high inductive reactance choke in the welding circuit, consumes energy and prevents accurate tracking of feedback profile. There is a reduction in the spatter at the fuse explosion; however, tracking by the D.C. chopper of the selected current profile during the remainder of the welding cycle is less accurate. For that reason, it has been suggested to employ a relatively small reactance choke in the welding circuit. In this manner, the premonition circuit actuated switch will drastically reduce the current at the fuse explosion and will also allow tracking of the feedback profile during other portions of the welding cycle. With the reduced choke size, the ripple factor of the welding current increases. Consequently, during the background portion of the cycle, especially after a plasma boost pulse, the ripple of the welding current can be such to extinguish the arc. When this occurs, the arc does not reignite until the next short circuit of the welding cycle has occurred. This causes some erratic behavior of the welding cycle which is to be avoided for smooth and quality welding. To overcome this difficulty, it has been suggested that a second power supply be provided, having a relatively low, fixed background current to maintain the minimum current available for the welding station at all times. Thus, the high ripple condition caused by use of a low inductive reactance choke will not diminish the available welding current below the background current from the second power supply. In this manner, a fixed background current is available at all times. The premonition circuit drops the welding current down to the background current by opening the premonition circuit switch and applying a snubber resistor in series with the choke. The low inductive reactance of the small choke allows accurate tracking of the profile by the feedback circuit whether it is controlled by voltage, current or wattage. However, the use of a separate and distinct power supply is expensive and as cumbersome as providing a relatively large choke. In addition, the background current from the second power supply was relatively high, i.e. in the neighborhood of 20 amperes.